Tin-lead solder plating has been conducted conventionally for connector terminals, semiconductor integrated circuit lead frames and the like. For the recent viewpoint of environmental protection, studies have been made on usage of tin (Sn) plating, tin-copper (Sn—Cu) alloy plating, tin-bismuth (Sn—Bi) alloy plating, tin-silver (Sn—Ag) alloy plating and the like not containing lead, as a substitute for tin-lead solder plating. A technique of Sn—Cu alloy plating is disclosed in Patent Document 1 described below.
If a coating film is made of the above-described alloy not containing lead, needle-like tin crystals called whiskers are likely to be generated. As whiskers are generated and grown, an electric short circuit failure may occur between adjacent electrodes. Since a whisker is as fine as about 1 μm and has a length of 1000 μm or longer in some cases, the whisker may be came off from the coating film and dispersed. Dispersed whiskers may cause a short circuit failure in or out of the apparatus.
One of the reasons of generating whiskers is an internal stress of a plated coating film. While tin recrystallizes by using the internal stress as a drive force, whiskers are grown. The internal stress of a plated coating film is accumulated by strain caused by lattice mismatch between an underlying metal film and a plated coating film, strain caused by eutectoid of additive of plating solution, strain caused by fine crystalline grains formed by brightener added to plating solution, and other strain. It is said that the larger the internal stress, i.e., strain, in a plated coating film is, recrystallization progresses in shorter time, and whiskers are likely to be grown.
The internal stress can be relaxed by non-gross plating which uses plating solution reducing brightener extremely or by semi-gross plating. It is confirmed that generation of whiskers can be effectively suppressed by adopting an approach to relaxing stress by performing heat treatment at about 150° C. after plating.
[Patent Document 1] JP-A-2001-26898